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Topic: Rivers 3
River Management Strategies
• Two types:
1.
Channelisation Strategies:
• River re‐alignment
• River re‐sectioning
2.
Bank protection strategies:
• Dykes/ Embankments
• Vegetation planting/
clearing
• Effectiveness of strategies
River Channel Management
Why manage river channels?
Flooding
occurs when:
1.
Clearance of vegetation increase surface runoff sudden increase in river volume
2.
Period of heavy and continuous rainfall river exceeds carrying capacity
These may be triggered by:• Global warming
– Extreme weather phenomena called El Nino and La Nina effect
HENCE THERE IS A NEED TO MANAGE
RIVERS. QUESTION IS, HOW?
River Channel Management Strategies
Two types of strategies1.
Channelisation Strategies:
• River re‐alignment
• River re‐sectioning
2.
Bank protection strategies:
• Dykes/ Embankments
• Vegetation planting/ clearing
Change the river
channel shape/
length, etc.
Make changes to
the river banks
1. Channelisation Strategies
• River re‐alignment
• River re‐sectioning
1.1 River Re‐alignment
• Straightening of river channel to increase speed
of river water to flow away from an
area more quickly & wash away sediments accumulated on river bed reduce flooding in that area
Source:
http://www.defra.gov.uk/enviro
n/fcd/policy/mrcomp/Image6.gi
f
River re‐alignment
Straightening of river = river re-alignment
E.g. Mississippi River (USA) has been shortened by 240 km to reduce threat of flooding.
• Widening
and deepening
of river channel.• Increase channel’s ability to hold waterdecrease likelihood of floods
Or,• Smoothening
of river beds
and banks
with
cement and granite less friction allow water to flow away more quickly reduce likelihood of floods
• E.g. Singapore River ay Boat Quay
1.2 River Re‐sectioning
River re‐sectioning
Widen & Deepen river = River Re- Sectioning
2. Bank Protection Strategies
• Dykes/ embankments
• Vegetation planting/ clearing
1.
Dykes/ Artificial levees : Walls of sand, stone or concrete built along rivers increase
river capacity to hold water reduce flooding
2.
Protects banks from erosion by force of running water reduced sediments flowing
into river increase river capacity less flood
• E.g. Artificial embankments along river of Pasir
Ris
Park, Singapore
2.1 Bank Protection (Embankments)
Embankments: Dykes/ Levees
Dykes
Dykes
• Roots of trees and plants holds soil together stabilise river banks
• Vegetation also slows down
rate of surface runoff prevent large amount of water entering river
at any one time• Reduced
surface runoff less sediments
transported river channel does not become shallower reduce flood occurrence
• E.g. Joint effort to protect mangroves along Mekong River.
2.2.1 Vegetation Planting
Vegetation Planting
• Weight
of vegetation may add stress to river banks
and cause them to collapse
• Fallen trees or branches in the river bed cause obstruction
to river flow
– Reduce speed of flow of river – Increase
likelihood of flooding
• E.g. Vegetation clearance done on Geylang River near Tanjong
Rhu, Singapore.
2.2.2 Vegetation Clearing
Vegetation Clearing
WHAT ARE THE LIMITATION(S)
OF THIS STRATEGIES?
Limitations of River Channel Management Strategies
1.
Causes flooding downstream
2.
Accumulation of sediments
3.
Loss of marine life and wetlands
4.
High cost of building and maintenance
1. Flooding downstream
Increase in speed of river due to channelisation
may result in flooding
downstream
Hence, flooding is not reduced. They can still occur in another part of the river.
Effectiveness of River Channel Management Strategies
Recall: River re‐alignment
strategy
Increase in speed of flow due to re- alignment upstream may cause flooding downstream.
Recall: River re‐sectioning
strategy
Increase in speed of flow due to re- sectioning upstream may cause flooding downstream.
2.1 Accumulation of sediments
• Channelisation
allows rivers to flow at higher speed.
• Sediments, previously deposited on river bed are carried downstream into sea/ ocean.
• Result: less sunlight reach seabed, affect growth of marine life such as coral reefs
Effectiveness of River Channel Management Strategies
Recall: River re‐alignment
strategy
Accumulation of sediments
2.2 Accumulation of sediments
• Bank protection measures such as dykes causes sediments to accumulate behind
these
structures
• Sediments build‐up over time river channel become shallower still flood with continuous rain
• Higher dykes need to be built/ money spent to dig out accumulated sediments
Effectiveness of River Channel Management Strategies
Recall: Building of dykes
Sediments accumulated behind structures makes the river shallower
E.g. Build up of sediments along lower course of Yellow River, China, making the channel shallower over the years.
3. Loss of marine life and wetlands1.Too much vegetation prevents sunlight from
reaching river bed kill aquatic life affect aquatic food chains
2.Changing river course, lining of banks with concrete destroys plants and animal habitats
3.Channelisation
leads to disappearance of wetlands. i.e. river flows over smaller area with
shortening of river course
Effectiveness of River Channel Management Strategies
Recall: River re‐alignment
strategy
Loss of wetlands
Loss of wetlands
4. High cost of building and maintenance
• Expensive to build and maintain river defenses
• Resources and manpower needed for river realignment and re‐sectioning.
• Aesthetically unappealing
Effectiveness of River Channel Management Strategies
In summation…Name of strategy/
Limitation(s)
Flooding
Downstream
Accumulation
of Sediments
Loss of
wetlands &
marine life
Expensive to
build &
maintain
Channelisation
Re‐alignment √ √(downstream)
√ √
Re‐sectioning √ √(downstream)
√ √
Bank protection
Building of
embankments
√
(behind
structures)
√
Clearing Vegetation √
Planting Vegetation √
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